Question: Find the largest positive integer $n$ such that
\[\sin^n x + \cos^n x \ge \frac{1}{n}\]for all real numbers $x.$
Answer: Setting $x = \pi,$ we get
\[(-1)^n \ge \frac{1}{n},\]so $n$ must be even.  Let $n = 2m.$

Setting $x = \frac{\pi}{4},$ we get
\[\left( \frac{1}{\sqrt{2}} \right)^{2m} + \left( \frac{1}{\sqrt{2}} \right)^{2m} \ge \frac{1}{2m}.\]This simplifies to
\[\frac{1}{2^{m - 1}} \ge \frac{1}{2m},\]so $2^{m - 2} \le m.$  We see that $m = 4$ is a solution, and the function $2^{m - 2}$ grows faster than $m,$ so $m = 4$ is the largest possible value of $m.$

We must then prove that
\[\sin^8 x + \cos^8 x \ge \frac{1}{8}\]for all real numbers $x.$

By QM-AM,
\[\sqrt{\frac{\sin^8 x + \cos^8 x}{2}} \ge \frac{\sin^4 x + \cos^4 x}{2},\]so
\[\sin^8 x + \cos^8 x \ge \frac{(\sin^4 x + \cos^4 x)^2}{2}.\]Again by QM-AM,
\[\sqrt{\frac{\sin^4 x + \cos^4 x}{2}} \ge \frac{\sin^2 x + \cos^2 x}{2} = \frac{1}{2},\]so
\[\sin^4 x + \cos^4 x \ge \frac{1}{2}.\]Therefore,
\[\sin^8 x + \cos^8 x \ge \frac{(1/2)^2}{2} = \frac{1}{8}.\]We conclude that the largest such positive integer $n$ is $\boxed{8}.$